The SUPPRESSOR of MAX2 1 (SMAX1)-Like SMXL6, SMXL7 and SMXL8 Act as Negative Regulators in Response to Drought Stress in Arabidopsis

Abstract

<jats:title>Abstract</jats:title> <jats:p>Drought represents a major threat to crop growth and yields. Strigolactones (SLs) contribute to regulating shoot branching by targeting the SUPPRESSOR OF MORE AXILLARY GROWTH2 (MAX2)-LIKE6 (SMXL6), SMXL7 and SMXL8 for degradation in a MAX2-dependent manner in Arabidopsis. Although SLs are implicated in plant drought response, the functions of the SMXL6, 7 and 8 in the SL-regulated plant response to drought stress have remained unclear. Here, we performed transcriptomic, physiological and biochemical analyses of smxl6, 7, 8 and max2 plants to understand the basis for SMXL6/7/8-regulated drought response. We found that three D53 (DWARF53)-Like SMXL members, SMXL6, 7 and 8, are involved in drought response as the smxl6smxl7smxl8 triple mutants showed markedly enhanced drought tolerance compared to wild type (WT). The smxl6smxl7smxl8 plants exhibited decreased leaf stomatal index, cuticular permeability and water loss, and increased anthocyanin biosynthesis during dehydration. Moreover, smxl6smxl7smxl8 were hypersensitive to ABA-induced stomatal closure and ABA responsiveness during and after germination. In addition, RNA-sequencing analysis of the leaves of the D53-like smxl mutants, SL-response max2 mutant and WT plants under normal and dehydration conditions revealed an SMXL6/7/8-mediated network controlling plant adaptation to drought stress via many stress- and/or ABA-responsive and SL-related genes. These data further provide evidence for crosstalk between ABA- and SL-dependent signaling pathways in regulating plant responses to drought. Our results demonstrate that SMXL6, 7 and 8 are vital components of SL signaling and are negatively involved in drought responses, suggesting that genetic manipulation of SMXL6/7/8-dependent SL signaling may provide novel ways to improve drought resistance.</jats:p>

Contribution of phenylpropanoid metabolism to plant development and plant–environment interactions

“…The smxl6smxl7smxl8 triple mutant exhibits significantly induced expression of genes encoding R2R3‐MYB TFs such as PAP1, PAP2, MYB113, and MYB114 which are responsible for the activation of anthocyanin‐biosynthesis genes (e.g., F3'H , DFR , and ANS ). Accordingly, the anthocyanin content is much higher than in wild‐type A. thaliana (Yang et al, 2020; Wang et al, 2020c).…”

Transcriptional Regulation of Drought Response in Arabidopsis and Woody Plants

“…Crosstalks between the ABA pathway and other pathways have been revealed to regulate drought response. For example, SMXLs from SL signaling are negatively involved in drought response by regulating both SL-responsive genes and ABAresponsive genes (Yang T. et al, 2020). Accumulated evidence supports the physical interactions among TFs from ABAdependent and ABA-independent pathways.…”

“…SUPPRESSOR OF MORE AXILLARY GROWTH2 (MAX2)-LIKE6 (SMXL6), SMXL7, and SMXL8 belonging to SAMX1-LIKE (SMXL) family, in addition to acting as transcriptional repressors in strigolactone signaling, negatively regulate drought response. Transcriptomic and physiological evidence suggested that these three SMXL proteins regulate drought response in both the ABA-dependent and ABAindependent pathways (Wang et al, 2020;Yang T. et al, 2020; Figure 1).…”

Crosstalk between phytohormones and secondary metabolites in the drought stress tolerance of crop plants: A review

“…It was observed that SL signaling mutants are hypersensitive to drought (Zhang et al, 2018). In tomato, even a minute level of SL in roots acts as a systemic signal for aerial tissues under drought stress (Visentin et al, 2016 (Yang, Lian, et al, 2020). Due to the antagonistic nature of CKs, it is speculated that SLs and CKs signaling might be crosstalking for regulating drought tolerance (Li, Herrera-Estrella, & Tran, 2019).…”

“…In a report, max2 mutant exhibits ABA hyposensitivity, thin cuticle, and ultimately compromised drought tolerance, which indicates that MAX2 or SL signaling is involved in positive regulation of drought stress tolerance (Bu et al, 2014; Van Ha et al, 2014). Recently it was reported that suppressors of MAX2 ‐ like (SMXL) 6, 7, and 8 negatively regulate drought tolerance by degrading MAX2 (Yang, Lian, et al, 2020). Due to the antagonistic nature of CKs, it is speculated that SLs and CKs signaling might be crosstalking for regulating drought tolerance (Li, Herrera‐Estrella, & Tran, 2019).…”